As is generally known, milk originating from a large number of animals flows into the collecting tank during machine milking. The resultant milk quality can be impaired strongly by milk originating from animals that are ill, especially from animals having an illness in an early stage which has not yet been diagnosed. For guaranteeing a high quality of the milk, it is therefore necessary to examine the milk during the milking operation and, if unsatisfactory quality is detected, to sort the milk out before it reaches the collecting tank.
Milk can be sorted out making use of the fact that the ion content of the milk of ill animals differs from that of the milk of animals that are not ill. Hence, the conductivity of milk can serve as a parameter on the basis of which milk of unsatisfactoy quality can be sorted out.
Known devices operating on the basis of conductivity measurements and used for separating the milk of ill animals are known from SU 961 609, DE 27 59 126 and WO 94/08450. According to the first-mentioned publication, the conductivity of milk is measured by means of two electrodes in the short milk hose between the teat cup and the collecting piece, and in said collecting piece the milk is separated into milk of satisfactory quality and mastitic milk. The flow conditions in the collecting piece are, however, of such a nature that, due to flow shade formation, only very inefficient cleaning by means of rinsing can be carried out. In addition, substantial vacuum losses will probably occur in the case of the embodiment shown so that this device is incable of fulfilling the requirements with regard to a stable vacuum on the teat. A collecting piece described in WO 94/08450 is subdivided into an upper and a lower chamber, the upper chamber being used for collecting the milk to be sorted out. Sensors for detecting parameters are provided on the inlet necks of the four short milk hoses between the teat cup and the collecting piece. Switching over is effected by means of valves rerouting the milk into the respective lower or upper chamber. Also in this device, problems will probably arise, when the device is being cleaned and disinfected in a rinsing circuit.
The use of the milking unit center as the location where the measurement is carried out is known from patent specification DE 1949559 among other publications. The center is subdivided into four chambers having installed therein the conductivity electrodes. Another embodiment is described in European patent EP 0054915. The center comprises four sub-chambers in which part of the milk of each quarter is collected and the conductivity is determined. For achieving an exchange of the milk in the chambers, outlet openings are provided in the base of the measurement chambers. This has the effect that part of the milk always flows from the measurement chamber back into the collecting piece. A varying milk stream may, however, result in a varying filling level in the measurement chamber, and this varying filling level causes unprecise measurements, especially in the case of low milk streams. When the outlet of the measurement chamber clogs, the milk in the measurement chamber will hardly be exchanged any more. All these conductivity sensors have in common that an electric line must be provided in the treading area of the animal, and this may result in an increased susceptibility to faults.